Is there really such a thing as a blue moon?

Bill Monroe sang about a "Blue Moon of Kentucky" but what exactly is a blue moon? And why do blue moons only occur once every 2 to 2 1/2 years?

(Editor's Note: This article was originally published on December 18, 2009. We re-air it again in honor of August 2012's second full moon tonight. Your comments are welcome,but please be aware that authors of previously published articles may not be able to respond to your questions.)

Once in a blue moon is a phrase often used to indicate a rare occurrence of something. For example, "It might snow in south Florida once in a blue moon," or if you're living in the northern tier of states, "Once in a blue moon the mercury might reach 80° in February." The earliest usage of the phrase dates back to 16th century England when William Barlos, Bishop of Chichester, included the line "Yf they saye the mone is belewe, We must believe that it is true," (If they say the moon is blue, We must believe that it is true) in one of his treatises. [1]

When folks say something might happen "once in a blue moon," it's understood that the incident has about as much of a chance to occur as a tropical palm has of sprouting in the landscape of my zone 5 garden. What about the colorization of the moon, specifically the full moon? I haven't checked the official records but my guess is that a full moon has never been blue in the literal sense. On occasion, the upper atmosphere could contain a high amount of dust particles which might render a slightly blue tint to the moon due to the process of light refraction. Records of this particular condition show that during the eruption of Krakatoa in 1883 there was so much smoke and dust particles in the atmosphere that the moon "appeared blue for nearly two years." [2]

Is this really a blue moon?

If you've an ear for bluegrass music you might be familiar with Bill Monroe's popular song "Blue Moon of Kentucky," it's one of the top attention-getters in the repertoire of songs my bluegrass band "Linefork" performs. The "blue" referred to in this song is a condition of woe, or sadness due to a lost companionship. I'm sure you know the feeling, I'm taken back to my high school days and a certain cheerleader.

When two full moons occur during the same calendar month (if you're keeping records, you'll want to mark December 2009 as a blue moon month), the second of these is usually referred to as the blue moon. Most years will have twelve full moons and these lunar cycles occur monthly. At the end of the regular calendar year there's a buildup of around 11 days and it's the accumulation of these extra days over a period of two or three years that gives us an extra full moon. [3]

December, 2009 will bring an end to the blue moons season with the second of its full moons occurring on December 31st (December had its first full moon on the 2nd day of the month). We'll have to wait until August of 2012 before we see the next blue moon season.

Orangish moons are quite common.

Blue moon statisticians sometimes list dates of blue moons for farmers; in particular the Maine Farmers' Almanac has kept certain moon info available to farmers and gardeners. Their listing of moon dates includes specific names for the varying moon phase, besides the blue moon. These lunar cycles are further divided into yearly quarters with associated moon phases: Mid Winter, Late Winter, Early Spring, Mid Spring, Late Spring, Early Summer, Mid Summer, Late Summer, Early Fall, Mid Fall, and Late Fall moons. If you're interested in knowing the dates of upcoming blue moons, there's an iPhone application, no, wait, it's not available there yet, but it is available on the Web - click here to check the dates. And if you or anyone you know gardens by the light of the full or blue moon, I would be interested in knowing if once in a blue moon, or on a more regular basis, you have an exceptionally productive season.